1. Field of the Invention
The present invention relates to solid-state image devices including a plurality of photoelectric converters for converting incident light into electric signals.
2. Description of the Related Art
In image pickup apparatuses, such as digital cameras including a solid-state image device, in order to make signal outputs of the solid-state image device more tolerant to disturbance noise, an external amplifier is disposed in close vicinity to the solid-state image device and amplifies the outputs. This structure will be described with reference to FIG. 11. Reference numeral 1 denotes a solid-state image device. Pixels each including a photoelectric converter are arranged two-dimensionally in a pixel area 2. An output amplifier 3 outputs signals of the pixels. An external amplifier 5 is disposed near an output terminal 4. Reference numeral 6 denotes a current source of the output amplifier 3. A power-save switch 7 switches the current consumption of the output amplifier 3. Switching the current consumption of the output amplifier 3 using the power-save switch 7 suppresses an increase in the dark current of a photoelectric converter located near the output amplifier 3, the increase being caused by heat generation of the output amplifier 3.
In addition, Japanese Patent Laid-Open No. 6-189065 discloses an image sensor including an output amplifier provided with a power-save mode.
For digital cameras, since waste of batteries depends on current consumption, reduction in the current consumption is considered in terms of increasing battery longevity.
However, due to the recent advancement of technologies, the signal-to-noise ratio of signal outputs have been improved, and new technological problems have occurred. In other words, when long-time accumulation over several seconds for astronomical photographs or the like is performed, nonuniformity of dark currents occurs due to heat generation or light emission of the internal amplifier and the external amplifier. In particular, when an external amplifier is disposed outside a sensor chip (solid-state image device) and near the solid-state image device, since the external amplifier generates heat, an output of a photoelectric converter located near the external amplifier increases due to a dark current. Thus, a problem in which dark current nonuniformity occurs on a screen, which is unique to solid-state image devices, occurs. This problem is more severe than maintaining known battery longevity. This is because an extremely long time, such as several minutes or several tens of minutes, is required for accumulation.
More specifically, when the temperature of a portion of the pixel area 2 near the external amplifier 5 increases by one degree due to heat generation of the internal amplifier and the external amplifier, a dark current nonuniformity of approximately 10% occurs. FIG. 12 shows generation of dark current nonuniformity. As shown in FIG. 12, a pixel output in an oblique line portion of the pixel area 2 increases due to heat generation of the external amplifier 5. This problem has a large effect on the quality of an image. FIG. 13 shows a case where the external amplifier 5 is mounted on the rear face of a package of the solid-state image device 1. Since the external amplifier 5 is mounted on the backside of the pixel area 2, an influence of dark current nonuniformity due to heat generation further increases. FIG. 14 shows another case where the external amplifier 5 is mounted on the rear face of the package of the solid-state image device 1. Since the package of the solid-state image device 1 is made of ceramics, a high thermal conductivity is achieved and an influence of dark current nonuniformity is increased.
In addition, for the internal amplifier, depending on the operation voltage of a transistor used, dark current nonuniformity occurs even when a current of several tens of microamperes flows. This is because a slight amount of infrared light is generated in the transistor due to an impact ionization phenomenon. In other words, only reducing currents, such as simply turning off a constant-current source, cannot prevent nonuniformity. This problem cannot be handled by known ideas.
In addition, when the power sources of the internal amplifier and the external amplifier are simply turned off during an accumulation time in order not to generate dark current nonuniformity, a phenomenon, such as an output of a sensor exceeding the rated voltage of the external amplifier, occurs depending on the circuit system. This causes the impact ionization phenomenon, and thus increasing the dark current nonuniformity.
Specific explanations will be given with reference to FIGS. 15 and 16.
FIG. 15 shows a constant-current circuit used for an amplifier in which an output current is determined by the voltage applied to the gate terminal of a transistor Q7 and the resistance of a resistor R3.
For example, when the voltage of the gate terminal is 2V and the resistance of the resistor R3 is 20 KΩ, a current of 100 μA is generated. A current is supplied from a common current source for a chip to a terminal IN, and the circuit shown in FIG. 15 generates a current value suitable for each circuit block. When the current input to the terminal IN is stopped in order to reduce the current of the entire chip, the total current value is reduced. However, although the gate voltage of a transistor Q12 during operation is about 3V, the gate voltage of the transistor Q12 increases to approximately 4.3V to 5V when the current from the terminal IN stops.
As a result, the amount of current flowing to the transistor Q12 increases. Thus, significant dark current nonuniformity due to an impact ionization phenomenon or the like may occur.
In addition, when a circuit shown in FIG. 16 is used for a buffer provided in the final output stage, a current is supplied from the common current for the chip to the terminal IN. The output buffer must be a high-speed and high-dynamic-range buffer.
Thus, a large current consumption and a power supply voltage of more than 5V, for example 8V, may be used. Since reduction in the current consumption of a high-speed amplifier is very effective, when the current input to the terminal IN is stopped in order to reduce the current of the entire chip, the total current value is reduced. However, the output terminal of the transistor Q12 is pulled up to a VDD side. As a result, a voltage that exceeds the rated voltage is input to the external amplifier.